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setState 的λης statewide method of Thread class in Java determines the current state of a thread. Thread has six possible states: NEW, RUNNABLE, BLOCKED, WAITING, TIME_WAITING, and TERMINATED. This article explains these states, their transitions, and how to control thread states through Thread APIs.

1. State Definitions

• NEW: Created thread before calling start()
• RUNNABLE: Ready to execute (justified as a combination of ready and running states)
• BLOCKED:_Thread waiting on a lock (synchronized)
• WAITING: Thread waiting for a notification
• TIME_WAITING: Waiting for timeout
• TERMINATED:Thread finished execution

Understanding these states helps in managing concurrent applications effectively.

2. Thread State Transitions

1. Initial State (NEW)

Upon creating a thread, it enters the NEW state. To switch to runnable, call start(). This transition is immediate upon start() invocation.

2. Runnable State

• Thread enters RUNNABLE state after visible thread selection by scheduler

3. Blocked State

• Thread acquires a lock via synchronized keyword
• Blocked until lock release by another thread

4. Waiting State

• Thread calls wait() and releases lock
• Blocks until notify() is called
• No CPU time allocated

5. Time-Waiting State

• Thread calls wait(time) with timeout specified
• Release lock and waits until timeout or notify
• No CPU time allocated

6. Terminated State

• Normal completion of run()-Abnormal exit due to uncaught exceptions
• main thread exit

3. State Transitions and Execution

Ordered States: NEW → RUNNABLE → ... → TERMINATED

• States before RUNNABLE are immutable; cannot skip
• States after RUNNABLE can backtrack

Avoid direct state transitions where not possible; follow APIs for state changes.

4. Thread State Examples

Example usage of wait() and notify():

public class WaitingExample implements Runnable {public static void main(String[] args) {WaitingExample Reversible = new WaitingExample();Thread t1 = new Thread(Reverable);Thread t2 = new Thread(Reverable);t1.start();t2.start();try {Thread.sleep(3);} catch (InterruptedException e) {e.printStackTrace();}System.out.println("t1：" + t1.getState());System.out.println("t2：" + t2.getState());try {Thread.sleep(1400);} catch (InterruptedException e) {e.printStackTrace();}System.out.println("t1：" + t1.getState());System.out.println("t2：" + t2.getState());}@Overridepublic void run() {method();}private synchronized void method() {try {Thread.sleep(1000);wait();} catch (InterruptedException e) {e.printStackTrace();}}}

5. Tips for Writing Lightweight Code

• Avoid unnecessary locks
• Optimize sleep durations
• Consider thread pooling for better resource utilization
• Minimize long waits

6. Summary

Understanding thread states helps manage concurrent applications effectively. Use Thread API methods to control state transitions and design thread-safe code carefully. Avoid over-engineering; keep code simple and efficient.

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